I have thought about the possibility of a sea anchor for my R21 Classic, and have some reservations. As a previous sailor, I had a sea anchor but never had to deploy it. If I would have had to, it would have been from astern to slow the boat down. It would have been OK, because the stern was relatively well-protected from a boarding sea, and the small cockpit was self-bailing. Heaving-to under minimal sail or bare poles was actually a better tactic as it presented the forward quarter of the boat to the seas.
In Nellie May, I am not sure from where I would deploy the sea anchor. I hesitate to deploy from astern because the very open, non-self bailing cockpit would be pretty dangerous in those conditions. Deploying from forward might be better, but I would not want to always be backing down on the rudder as the stresses imposed might cause failure. Deploying in a manner similar to heaving-to would present the beam to the seas and the 21-footers are already pretty tender in that respect.
Maybe the only conditions where I would seriously consider deploying a sea anchor is where I lost my motor for some reason...and I think we all go to some extremes to prevent that from happening. Otherwise, present the bow to the seas at a slow-ahead speed and ride it out.
...be interested in what others think.